1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method as well as an electronic camera and a scanner, each of which utilizes the image processing apparatus and the image processing method. More specifically, the present invention relates to a noise reduction method or a noise reduction apparatus for reducing a nose of the image.
Priority is claimed on Japanese Patent Application No. 2005-131659, filed Apr. 28, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
An image processing apparatus generates high quality digital image data from image signals that are obtained by an image pickup element such as a charge coupled device (CCD). Typical examples of the generation of the high quality digital image data is a noise reduction process that reduces a noise included in an image. A noise included in the image may be generated by a variety of causes. Noises caused by an image pickup element are highly influential. Typical examples of the noises caused by the image pickup element include a dark current noise and a shot noise. The dark current noise is generated by heat while the image pickup element does not receive a light.
The dark current noise is almost constant over the position of an image. The dark current noise is superimposed over an ideal image of an object. This superimposition increases brightness of the image. The superimposition may cause “bright black” that has a non-zero black level.
The shot nose is generated by a probabilistic fluctuation that is caused by optoelectric conversion. The shot noise is one of random noises in the image. The quantity of the fluctuation is proportional to a square root of the number of photons. The amount of the shot noise increases as the number of photons increases or as the quality of light incident to the image pickup element increases. For example, when one hundred of the level of the output image signal is obtained by one hundred of the quantity of the incident light, ten of the level of the shot noise may be generated so that the level of the output image signal may vary in a range of 90-110. When the quantity of the incident light is 10000, the level of the shot noise may be 100, so that the level of the output image signal may vary in a range of 9900-10100.
In general, it is more difficult to reduce the shot noise than reducing the dark current noise. The shot noise is in general greater than the dark current noise. The shot noise is more influential to the image than the dark current noise.
As described above, the amount of the shot noise is correlated to the number of photons. The amount of the shot noise depends on not only the intensity of light but also the area of a single pixel of an image pickup element as well as characteristics of optoelectronic conversion of the image pickup element and properties of a color filter. The amount of the shot noise depends on the image pickup element.
Japanese Unexamined Patent Application, First Publication, No. 2001-157057 discloses the following techniques. Constant real numbers a, b, and c are given and a signal level D represents a concentration value. “a”, “b”, “c” and “d” are used to functionalize the amount of noise N with N=abcD. The amount of noise N to the signal level D is estimated. Characteristics of frequency of filtering are controlled based on the estimated noise N, thereby performing a noise reduction process appropriate to the signal level D.
Japanese Unexamined Patent Application, First Publication, No. 2001-175843 discloses the following techniques. The image signal is divided into a luminance signal and a chrominance signal. A smoothing process is performed to the chrominance signal other than a luminance edge and a color edge, thereby reducing a color noise. As a result, the color noise reduction can be obtained without deterioration of the resolution.